A straw tube drift chamber is used in the detection of secondary particles produced by accelerated particle collisions. These chambers consist of ionizable gas filled tubes with a conductive wire running lengthwise down the tube's center. The wire enclosed in the tube is under tension to maintain it in alignment within the tube.
The tube itself is made of conductive material (typically aluminized mylar laminated on a carbon composite film) and acts as the cathode of the cell when a high voltage is applied to the wire (anode). The tubes are small in diameter (on the order of 4 to 8 mm). The small size allows for arrays of more tubes in smaller areas, thus, providing detectors with higher resolution than can otherwise be obtained.
Large arrays of these thin straw tube chambers are configured about the collision point of a particle accelerator to detect and track collision products of the primary impact. These collision products are called secondary particles. As a secondary particle passes through the tube of the straw tube chamber, the gas is ionized and a trail of electrons migrate to the conductive wire. This trail of electrons provides a signal that a secondary particle has passed through the straw tube near that location. The signal is a measurable charge that is recorded by the instruments monitoring the straw tube chamber array.
Conventional technology utilized drinking straw apparatus and techniques to form straw tubes. These tubes are generally circular in cross section. After the tubes are formed, a conductive wire is threaded from one end of the tube to the other, tensioned and then fixed in position.
A number of universities and private organizations have conducted research in the area of straw tube production materials, size and resolution. One of the first array of straw tube chambers was called the HRS vertex chamber and was constructed at Indiana University in 1981. The chamber had an array of 356 circular tubes. Each of the tubes was 46 cm long with walls made of 85 micron thick aluminized mylar.
A similar chamber built at the University of Colorado, reportedly had an array of 640 eight millimeter diameter cells circular with a length of 84 cm. The walls of that cell were also made of aluminized mylar with a thickness of 75 microns.
Chambers were also built at other institutions. Normally, aluminized polycarbonate, aluminized mylar, or a composite of the two materials were used for the conductive tube with a wall thickness of 25 to 85 microns. The total number of cells were in the hundreds, the lengths were on the order of 40 to 60 cm, and the tube diameters ranged between 4 and 7 mm.
The length of the tubes is necessarily limited by the manufacturing apparatus and method, and the materials of construction. It is also limited by the strength and stiffness of the conductive wire within the tube.
The dimensions of the tubes are directly related to the resolution of the chamber. Smaller, longer tubes can lead to better resolution because they can utilize space more efficiently. However, the drinking straw manufacturing technology used to produce these straw tubes places limits on the dimensions. Similarly, since it is necessary to thread the tube with conductive wire, a certain minimum tube diameter must be maintained.
In addition, resolution is directly related to the shape of the tubes. When the shape allows for a tight packing density, more tubes can be positioned in a given area and detection of the passing particles can be measured at more locations. Under the present manufacturing technology, the tubes are generally circular. Therefore, when packed into an array, there are gaps in the array corresponding to the dead spaces therebetween.
Further, the threading process in the present method of manufacturing straw tubes is an additional limitation on the size of the tube arrays. It takes some time to position the conductive wire in the tube structure. In cases where thousands of completed tubes, not hundreds, are needed, the process becomes inefficient. This inefficiency provides incentive to limit the number of tubes used in an array, which limitation influences resolution.
With more and more emphasis being placed on the better resolution, the size and shape of straw tube detectors becomes increasingly important. The smaller the tube and the more the tube's shape allows for higher packing densities, the more tubes can be packed into a given space which results in a higher resolution. This increased sensitivity allows for the location of a penetrating particle to be pinpointed and tracked more accurately.
However, conventional straw making technology is not practical for mass production needs of extremely thin straw tube chambers.